We report the detection of a strong, organized magnetic field in the O9IV star HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at the 3.6 m Canada–France–Hawaii Telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Programme. From the fitting of our spectra using non local thermodynamic equilibrium model atmospheres, we determined that HD 57682 is a 17⁺¹⁹₋₉M_⊙ star with a radius of 7.0^+2.4_−1.8R_⊙ and a relatively low mass loss rate of 1.4^+3.1_−0.95× 10⁻⁹M_⊙yr⁻¹. The photospheric absorption lines are narrow, and we use the Fourier transform technique to infer v sini= 15 ± 3 kms⁻¹. This v sini implies a maximum rotational period of 31.5d, a value qualitatively consistent with the observed variability of the optical absorption and emission lines, as well as the Stokes V profiles and longitudinal field. Using a Bayesian analysis of the velocity resolved Stokes V profiles to infer the magnetic field characteristics, we tentatively derive a dipole field strength of 1680⁺¹³⁴₋₃₅₆G. The derived field strength and wind characteristics imply a wind that is strongly confined by the magnetic field.